Atmos. Chem. Phys., 11, 12227–12239, 2011
www.atmos-chem-phys.net/11/12227/2011/
doi:10.5194/acp-11-12227-2011
© Author(s) 2011. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Characterization of a volcanic ash episode in southern Finland
caused by the Grimsvötn eruption in Iceland in May 2011
V.-M. Kerminen1,2 , J. V. Niemi3,4 , H. Timonen1 , M. Aurela1 , A. Frey1 , S. Carbone1 , S. Saarikoski1 , K. Teinilä1 ,
J. Hakkarainen1 , J. Tamminen1 , J. Vira1 , M. Prank1 , M. Sofiev1 , and R. Hillamo1
1 Finnish
Meteorological Institute, Research and Development, P.O. Box 505, 00101, Helsinki, Finland
of Physics, University of Helsinki, P.O. Box 64, 00014, Helsinki, Finland
3 Helsinki Region Environmental Services Authority HSY, P.O. Box 100, 00066, Helsinki, Finland
4 Department of Environmental Sciences, University of Helsinki, P.O. Box 65, 00014, Helsinki, Finland
2 Department
Received: 30 August 2011 – Published in Atmos. Chem. Phys. Discuss.: 5 September 2011
Revised: 4 November 2011 – Accepted: 16 November 2011 – Published: 7 December 2011
Abstract. The volcanic eruption of Grimsvötn in Iceland
in May 2011 affected surface-layer air quality at several locations in Northern Europe. In Helsinki, Finland, the main
pollution episode lasted for more than 8 h around the noon of
25 May. We characterized this episode by relying on detailed
physical, chemical and optical aerosol measurements. The
analysis was aided by air mass trajectory calculations, satellite measurements, and dispersion model simulations. During the episode, volcanic ash particles were present at sizes
from less than 0.5 µm up to sizes >10 µm. The mass mean
diameter of ash particles was a few µm in the Helsinki area,
and the ash enhanced PM10 mass concentrations up to several tens of µg m−3 . Individual particle analysis showed that
some ash particles appeared almost non-reacted during the
atmospheric transportation, while most of them were mixed
with sea salt or other type of particulate matter. Also sulfate of volcanic origin appeared to have been transported
to our measurement site, but its contribution to the aerosol
mass was minor due the separation of ash-particle and sulfur dioxide plumes shortly after the eruption. The volcanic
material had very little effect on PM1 mass concentrations or
sub-micron particle number size distributions in the Helsinki
area. The aerosol scattering coefficient was increased and
visibility was slightly decreased during the episode, but in
general changes in aerosol optical properties due to volcanic
aerosols seem to be difficult to be distinguished from those
induced by other pollutants present in a continental boundary
layer. The case investigated here demonstrates clearly the
Correspondence to: V.-M. Kerminen
([email protected])
power of combining surface aerosol measurements, dispersion model simulations and satellite measurements in analyzing surface air pollution episodes caused by volcanic eruptions. None of these three approaches alone would be sufficient to forecast, or even to unambiguously identify, such
episodes.
1
Introduction
Volcanic eruptions perturb aerosol and trace gas budgets in
the troposphere and stratosphere, having thereby a potential
to influence the global climate, air traffic, soils and vegetation, as well as animals and humans. The effects of volcanic eruptions depend crucially on the magnitude, type and
height of emissions. Climatic effects by volcanoes remain
minor unless a large amount of sulfur dioxide is being injected to the stratosphere (Robock et al., 2000; Graf et al.,
2007; Schneider et al., 2009). Volcanic ash may cause serious damage to air craft (e.g. Guffanti et al., 2009; Gislason et
al., 2011), and even relatively moderate concentrations of ash
particles in the middle and upper troposphere may force air
traffic authorities to change flight routes or to cancel flights
(Prata, 2009; Schumann et al., 2011). Both gaseous and particulate emissions by volcanoes may have adverse effects on
the environment and human health (e.g. Thordarson and Self,
2003), but this is possible only when the emissions enter the
boundary layer by either direct air mass transport or sedimentation.
Surface air pollution and associated effects caused by volcanic eruptions have been investigated to a very limited
Published by Copernicus Publications on behalf of the European Geosciences Union.
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V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
extent. One of the most widely-documented cases in this
respect has been the Laki eruption that took place in Iceland during 1783–1784, causing serious environmental and
health problems over large areas in Europe and even farther away (Thordarson and Self, 2003). The eruption of
Eyjafjallajökull in Iceland during April–May, 2010, initiated a big number of research activities to characterize the
physical, chemical and optical properties of airborne volcanic material associated with this eruption. Over the continental Europe, the signal from the Eyjafjallajökull eruption
was strongest above the boundary layer (e.g. Ansmann et al.,
2010; Gasteiger et al., 2011; Schäfer et al., 2011; Schumann
et al., 2011), but it could also be detected in the surface air
and ground deposits (e.g. Colette et al., 2010; Flentje et al.,
2010; Lettino et al., 2011; Revuelta et al., 2011; Rossini et
al., 2011).
The main challenge in measuring volcanic material in the
surface air is that this material is always mixed with other
pollutants present in the boundary layer. In this paper, we
will document a prominent surface air pollution episode observed in Helsinki, Finland, caused by long-range transported volcanic material from the Grimsvötn eruption that
took place at the end of May 2011, in Iceland. The pollution
episode investigated here was observed on 25 May 2011, and
it was characterized in detail by a number of different measurement devices. In this paper, we aim to address the following two questions: (1) what were the physical and chemical properties of volcanic aerosol particles after being entrained into the boundary layer air, and (2) how well these
particles could be separated from other natural and anthropogenic aerosol particles present in measured air masses?
In addition to providing new information about long-range
transported volcanic aerosol particles, we will briefly discuss
the compatibility of our observations with satellite measurements and dispersion model simulations.
2
2.1
Materials and methods
Aerosol measurements
Excluding the particle number size distribution and
PM2.5 /PM10 data, the aerosol measurements discussed in
this paper were conducted at the urban background station
SMEAR III (Station for Measuring Ecosystem–Atmosphere
Relationships; 60◦ 200 N, 24◦ 970 E, 26 m above sea level) in
Helsinki, Finland. The SMEAR III station is located in the
heart of the Helsinki Metropolitan Area about 5 km northeast from the Helsinki city center. The station is surrounded
by university campus buildings, other buildings and small
forested areas. The closest major road is located behind a
few trees at the distance <200 m from the station (Saarikoski
et al., 2008; Järvi et al., 2009).
The PM1 mass concentrations were continuously measured using a Tapered Element Oscillating Microbalance
Atmos. Chem. Phys., 11, 12227–12239, 2011
(TEOM© 1400a, Rupprecht & Patashnick; Allen et al.,
1997). A cyclone (sharp cut cyclone SCC 1.829, BGI Inc.
US) was used prior to the TEOM to cut off large particles (aerodynamic diameter >1 µm). The uncertainty of the
TEOM results was estimated to be 10 %.
An aerosol chemical speciation monitor (ACSM) was used
to measure the mass and chemical composition (sulphate, nitrate, ammonium, chloride and organics) of non-refractory
submicron particulate matter. The time resolution of the
measurements was one hour. The ACSM is described in detail by Ng et al. (2011). Shortly, the sample flow rate into the
instrument (∼85 cm3 min−1 ) is fixed by a 100-µm-diameter
critical aperture. An aerodynamic lens is used to focus the
particle beam. The measured particle size is controlled by an
aerodynamic lens that has transmission >50% over the range
from 75 to 650 nm (Liu et al. 2007). Three vacuum pumps
(Pfeiffer, two HP300 and one HP80) and one backup diaphragm pump (Vacuubrand, MD1) are used to generate vacuum into the instrument. Focused particle beam impacts the
vaporizer (600 ◦ C) and formed vapor is ionized by electron
impaction. Compounds that are vaporized and ionized are
detected by a residual gas analyzer (RGA) type quadrupole
mass analyzer. The ACSM measures background automatically by alternating sample flow through a filter line and the
normal sample line. The particle signal is calculated by subtracting the background results (filter mode measurements)
from the sample results. The detection chamber contains a
naphthalene source, which provides an internal standard for
calibrating the mass to charge ratios of the measured ions.
Aerosol scattering and absorption coefficient were measured using a three-wavelength nephelometer (TSI Model
3563) and aethalometer (Magee Scientific, AE20), respectively. For both these instruments, air was collected both
through a PM1 inlet removing super-micron particles and
without inlet and size discrimination during the epidode.
After realizing that volcanic aerosol had probably entered
the Helsinki area on 25 May, we initiated impactor sampling
for chemical analysis by ion chromatography (IC) and individual particle analysis by transmission electron microscopy
(TEM) coupled with energy-dispersive X-ray (EDX) microanalyses. Between 11:37 and 15:05 local summer time
(= UTC + 3 h) on 25 May, five stages (the stages 1, 2, 3, 4
and 7 with cut-off diameters of 0.2, 0.5, 1.0, 1.8 and 11 µm,
respectively) of a single orifice Battelle type impactor (model
I-1, PIXE International Corporation, Florida, USA, modified from the original Battelle design, Mitchell and Pilcher,
1959) were used to collect samples for the TEM/EDX analysis (description in the next section). The sample collection with a volume flow rate of 1 liters per minute took
place onto 3-mm TEM grids (Cu with carbon-coated Formvar films, 400-mesh, Carbon Type-B, Ted Pella Inc., Redding, CA, USA). Between 15:15 and 17:00 on the same
day, the four highest stages (cut-off diameters of 1.0, 1.8,
3.2 and 5.6 µm) of a Micro-Orifice Uniform Deposit Impactor (MOUDI, Marple et al., 1991) were used to collect
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V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
supermicron aerosol particles for the IC analysis (description in the next section). The MOUDI samples were collected with a volume flow rate of 30 liter per minute onto
47-mm polycarbonate filters with 0.4 µm pore size (Isopore
membrane filters, Millipore). An inlet with a cut-off diameter of 10 µm (Liu and Pui, 1981) was used on the top of the
sampling line at about five meters above the ground level.
Particle number size distributions were measured as a part
of a technical test conducted at the ventilation room of an
elementary school (Ressu comprehensive school, Lapinlahdenkatu 10, Helsinki) in the Helsinki city centrum. Automatic valves were used to alternate the measurements between ambient air prior filter and ventilated air after filter.
Only data from ambient measurements are used in this article. Particle number size distributions were measured in
the size range of 9–414 nm with a scanning mobility particle sizer (SMPS, TSI Model 3080/3081) and in the size
range of 0.5–20 µm with an aerosol particle sizer (APS, TSI
Model 3321). In case of the APS, losses of coarse particles inside the ventilation channels and valves were plausible. By comparing the APS-derived mass concentrations
with the PM2.5 and PM10 mass measurements elsewhere in
the Helsinki area, these losses were estimated to be up to 50–
70%, mostly for particles >5 µm in aerodynamic diameter.
2.2
Analysis of collected aerosol samples
From the MOUDI filter samples, the major inorganic (Na+ ,
−
2−
+
2+
2+
−
NH+
4 , K , Mg , Ca , Cl , NO3 , SO4 ) water-soluble
ions were analyzed by using an ion chromatograph (Dionex
ICS-2000). The AG17/CG12A guard column, AS17/CS12A
analytical column, 500 µl, ASRS/CSRS ultra II suppressor
and KOH/MSA eluent were used to analyze anions/cations.
The uncertainty of the IC analyses of the filters was about
5–10 % for all of the analyzed ions.
The morphology and elemental composition of individual
aerosol particles were investigated using a Tecnai 12 transmission electron microscope (TEM) equipped with an EDAX
energy dispersive X-ray (EDX) microanalyser. The TEM
was operated on an accelerating voltage of 120 kV and with a
low beam current to minimize beam damage. Counting time
for X-ray spectra was 20 live seconds. The minimum size
of particles analyzed was 0.2 µm (geometric diameter). The
elements analysed were from C to Pb, excluding N. The Xray counts from carbon-Formvar coating (thickness listed as
60 nm; contains abundant C, some O and very minor Si) of
TEM grids were estimated by analyzing blank areas between
particles. The X-ray count ratios of elements for each particle were normalized to 100%. Although the elemental results
were semi-quantitative, the accuracy is sufficient to identify
different particle types and to compare the differences in elemental ratios of the same particle type in different samples
(see Niemi et al., 2006, and references therein). The strong
vacuum (10−7 torr) and beam exposure causes evaporation
of semi-volatile compounds from particles, and for that reawww.atmos-chem-phys.net/11/12227/2011/
12229
son water, ammonium nitrate and organic compounds with
high vapour pressures were lost, as is typical in electron microscopy.
2.3
Satellite measurements
In order to study the ash plume arrival to Finland we have
used the observations retrieved from the measurements made
by the Dutch-Finnish Ozone Monitoring Instrument (OMI)
on-board NASA’s polar orbiting EOS-Aura satellite. OMI
measures scattered solar light in UV-VIS wavelengths and
several trace gases (including O3 , NO2 , SO2 , HCHO and
BrO), aerosols and surface UV-radiation can be retrieved
from the measurements (Levelt et al., 2006). The ground
pixel size is at best 13×24 km in nadir and larger at swath
ends. Since 2009 the so called OMI row anomaly has degraded the global coverage which is now obtained in two
days instead of daily coverage in the beginning of the mission
in 2004. Naturally, this has also affected OMI’s capability to
monitor volcanic ash and SO2 .
There are two aerosol products from OMI: OMAEROUV
and OMAERO. The first one continues the traditional Total
Ozone Mapping Spectrometer (TOMS) aerosol products and
the latter one is a multi-wavelength product taking advantage
of the wider spectral region of OMI (Torres et al., 2007). The
ash and SO2 emitted from volcanoes have been successfully
seen in OMI SO2 and UV Aerosol Index products. The OMI
SO2 retrieval technique is based on band residual difference
algorithm defined in Krotkov et al. (2006). In this work we
have used the middle troposphere (TRM) SO2 product that
assumes the center of mass altitude to be at 7.5 km.
In addition to SO2 , we have used the OMI Absorbing
Aerosol Index (AAI) which is a qualitative measure that indicates the presence of UV-absorbing aerosols, such as smoke,
dust and volcanic ash, in the atmosphere. Originally, it was
developed empirically from TOMS observations (Torres et
al., 1998). Since the algorithm is simple and robust and does
not take assumptions on aerosol microphysics and compositions, AAI has been used in numerous studies of transported aerosols (see, e.g. Dirksen et al., 2009). The UV
aerosol index is essentially a measure of scattering compared
to fully molecular atmosphere with only Rayleigh scattering. The AAI is the positive part of the UVAI values, while
negative UVAI values indicate non-absorbing aerosols and
clouds. The UVAI is close to zero when the atmosphere is
free from aerosols. The advantage of the AAI is that it is useful indicator of aerosols over large variety of land surfaces
as well as above sea (with the exception of snow covered regions) (Torres et al., 2007). Compared to the OMI Aerosol
optical depth (AOD) product that is sensitive to clouds, the
AAI, while not a geophysical quantity, is also useful when
the aerosols are present above clouds. OMI scientific data
are publicly available from NASA’s Mirador Earth Science
Data Search Tool and near real time (NRT) data are available
from NASA’s LANCE and KNMI’s TEMIS services. OMI
Atmos. Chem. Phys., 11, 12227–12239, 2011
V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
2.4
Other materials and methods
The total mass concentrations of aerosol particle smaller than
2.5 µm (PM2.5 ) and 10 µm (PM10 ) in diameter are continuously measured at several sites in the Helsinki Metropolitan
Area by the Helsinki Region Environmental Services Authority (HSY). The results from Kallio urban background
station were selected for this work since Kallio station is located close (<2 km) to SMEAR III station and the same PM
monitor type (TEOM© 1400a) is used at both stations. We
also utilized PM results from other monitoring sites in Finland (Finnish national air quality portal; www.ilmanlaatu.fi),
Norway (Norwegian national air quality portal; http://www.
luftkvalitet.info) and Sweden (Stockholm – Uppsala County
Air Quality Management Association; www.slb.nu) to obtain
general overview on the spatial distribution and timing of the
episode.
Basic meteorological quantities, including visibility, and
selected trace gas concentrations are continuously measured
at the SMEAR III station (e.g. Järvi et al., 2009). Air mass
back trajectories for the time period considered here were
determined using the NOAA hysplit model (Draxler and
Rolph, 2011; Rolph, 2011). Trajectories were calculated for
72 h backward in time at three arrival heights (0, 500 and
3000 m a.s.l) and arrival time of 00:00 UTC each day.
Dispersion of volcanic emissions was simulated with the
System for Integrated modeLling of Atmospheric coMposition SILAM (Sofiev et al., 2006, 2008). The dynamic core
of the model is based on the transport scheme of Galperin
(1999, 2000) combined with the extended resistance analogy of Sofiev (2002) for vertical diffusion. The removal processes are described via dry and wet deposition. Dry deposition follows Slinn and Slinn (1980). The SILAM wet
deposition parameterization (Sofiev et al., 2006; Horn et al.,
1987; Smith and Clark, 1989; Jylhä, 1991) is based on direct
observations for moderately hydrophobic aerosols. For the
current study, the meteorological information was taken from
the ECMWF meteorological model (http://www.ecmwf.int).
Diagnosing the features of the boundary layer and the free
troposphere followed Sofiev et al. (2010). The resolution
of the simulations was 0.25◦ with 19 vertical layers up to
about 20 km above the surface. The emission source was defined by the plume top altitude as reported by the London
Volcanic Ash Advisory Centre based on the observations of
the Icelandic Meteorological Office. The mass flux was estimated from the eruption height using the relation of Mastin
et al. (2009). The particle size spectrum was represented as
a single bin centered at the diameter of around 3 µm. This
fraction was assumed to comprise 1 % of the eruption total
mass flux.
Atmos. Chem. Phys., 11, 12227–12239, 2011
100
-3
direct broadcast data over Europe, Iceland and Greenland are
available from OMI Very fast delivery site.
PM concentration (µg m )
12230
80
PM10
PM2.5
PM1
60
40
20
0
22/5/11
23/5/11
24/5/11
25/5/11
26/5/11
27/5/11
28/5/11
Fig. 1. PM1 mass concentration measured at the SMEAR III station, together with PM2.5 and PM10 mass concentrations measured
at another site (Kallio) in Helsinki between 22 and 28 May 2011.
3
3.1
Results and discussion
General overview of the episode
The pollution episode caused by the volcanic eruption was
observed around the local noon on 25 May 2011 in the
Helsinki area (Fig. 1). The highest PM10 mass concentrations (>100 µg m−3 ) were observed between about 09:00 and
10:00, after which there was a small dip in concentrations
followed by a second maximum a couple of hours after the
noon. Altogether, PM10 mass concentrations remained above
50 µg m−3 for more than 8 h in Helsinki. The great majority of the elevated particle mass concentrations could be explained by coarse particles (particle aerodynamic diameter
between 2.5 and 10 µm), since the PM2.5 mass concentrations remained mainly below 20 µg m−3 and PM1 mass concentrations did not show any noticeable increases during the
episode.
In order to put our observations into a broader context,
we could note that annual-average PM10 mass concentrations
are about 20 µg m−3 in the Helsinki area (Anttila and Salmi,
2006). The vast majority of short-term PM10 mass concentration peaks in Helsinki, like in several other northern cities,
can be ascribed to traffic-induced dust formation because of
the use of studded tyres and gravelling as antiskid treatments
(Anttila and Tuovinen, 2010; Kupiainen and Pirjola, 2011).
Road dust episodes are most frequent during the snowmelt
period in spring, and similar to the episode considered here,
they are seen most clearly in the coarse-particles size fraction. Air pollution episodes characterized by elevated PM2.5
mass concentrations are also observed in the Helsinki area
every year. Such episodes are usually caused by long-range
transported anthropogenic pollution or wildfire smokes from
Eastern Europe and sometimes also by local wood burning
activities or traffic exhaust (e.g. Vallius et al., 2003; Sillanpää
et al., 2005; Saarikoski et al., 2008; Niemi et al., 2009). The
long-range transport episodes of coarse particles are very rare
in Finland (Tervahattu et al., 2004).
According to local wind measurements and air mass
back trajectory calculations, westerly air masses prevailed
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V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
12231
Fig. 2. The AAI index measured by OMI during the afternoon on 22, 23, 24 and 25 May 2011.
between 22 and 28 May 2011. During the episode on 25 May,
measured air masses originated from Northern Atlantic and
typically traversed over the very southern parts Norway and
over Sweden before entering Helsinki across the Baltic Sea.
The average wind speed was relatively high between about
7 and 8 m s−1 during the episode, and the average temperature was in the range of 10–12 ◦ C at the SMEAR III site in
Helsinki.
The eruption of Grimsvötn started on 21 May, was at its
strongest on 22 May, and then rapidly weakened during the
next few days. Figure 2 shows the OMI AAI index during
four days prior to the observed air pollution episode at the
times of the satellite overpass. Indicative of ash emitted by
Grimsvötn, we may see very large values of the AAI index
over Iceland on 22 May and similarly high values South of
Iceland in the following day. On 24 May the main part of
the volcanic ash cloud approached Southern Norway. On
25 May this cloud was substantially diluted and the largest
values of AAI could be observed over the Baltic Sea and Estonia, as well as over the southern parts of Finland. During
the air pollution episode, OMI overpassed southern Finland
twice (Fig. 3). The AAI indices measured during these two
overpasses give strong support for the presence of a volcanic
cloud over the Helsinki area during most of the episode, even
though the thickest part of the cloud was probably located
somewhat south of our measurement site.
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It is interesting to note that high PM10 mass concentrations were measured at several sites in Southern Norway on
24 May. In Stockholm, Sweden, PM10 concentrations started
to increase during the late evening of 24 May and the episode
lasted until the morning of 25 May. Besides the Helsinki
area, the PM10 episode could be detected in several other locations of the very southern parts of Finland during 25 May.
The PM10 and satellite measurements along with back trajectory calculations indicate strongly that the episode observed
in Helsinki on 25 May was due to volcanic material ejected
by Grimsvötn two to three days earlier, and that the same
material affected also parts of Norway and Sweden.
Compared with the AAI index, the SO2 concentration retrieved by OMI showed a very different dispersion pattern.
The main SO2 plume was located over Iceland on 22 May,
after which it travelled over Greenland on 23 May and was
then split into at least two separate plumes. On 24 May
the largest SO2 concentrations were observed as about 300km-wide belt that grossed Greenland and reached Northern
America, and as a narrow stripe that went over the Arctic
Ocean, Spitsbergen and Northern Siberia. On 25 May largest
SO2 concentrations were observed to the west of Greenland,
while some remains of the plume could be detected over different parts of Northern Atlantic and Arctic Ocean. The separation of volcanic ash and SO2 shortly after the eruption
has been observed several times before (e.g. Schneider et al.,
Atmos. Chem. Phys., 11, 12227–12239, 2011
12232
V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
Fig. 3. The OMI AAI values >1 during the two OMI overpasses that took place on 25 May, together with MODIS/Aqua true color images.
The MODIS granules are acquired at 10:30 and 12:05 UTC, and the OMI overpassed the same location ∼7 min later.
1999; Prata and Kerkmann, 2007). Such a separation might
be caused by potential differences in the timing and ejection
heights of SO2 and ash particles emissions, or by the sedimentation of ash particles from the dispersing plume. The
main practical consequence of this phenomenon is the presence of substantially less sulfate and fewer submicron particles than there would have been if SO2 and ash particles had
been transported together.
Yet another way of investigating the transport of volcanic
ash to our measurement site is to simulate it using a dispersion model. The simulated total aerosol column (Fig. 4)
shows the volcanic plume reaching the northern Europe in
24 May with the highest column loadings dispersed over the
North Sea and later, Southern Scandinavia and Baltic Sea.
The simulated plume reaches Southern Finland during the
25 May. With the currently available input information, the
model could not resolve the observed differences in the dispersion patterns of SO2 and ash, and the modeled aerosol
column therefore has features of both observed AAI and SO2
column.
3.2
Aerosol size distribution, chemical composition and
optical properties
The PM1 , PM2.5 and PM10 measurements discussed in
Sect. 3.1 indicate that the pollution episode observed on
25 May in the Helsinki area was mainly due to increased concentrations of super-micron (diameter >1 µm) particles. Our
number size distribution measurements give strong support
for this view. The mass concentrations of super-micron particles derived from APS number size distributions increased
Atmos. Chem. Phys., 11, 12227–12239, 2011
several-fold during the episode with a peak diameter between
about 3 and 4 µm (Fig. 5). The real mass mean diameter
of super-micron particles could have been somewhat bigger
than this because of losses of >5 µm particles in our measurement system. Compared with other periods between 22
and 28 May, sub-micron particle number size distributions by
the SMPS measurements did not show any specific features
(data not shown). This is contrary to the Eyjafjällajökul eruption in April 2010, the plumes of which contained large numbers of sub-micron particles, presumably as a result of newparticle formation associated with the emitted SO2 (Flentje
et al., 2010; Petäjä et al., 2011; Schäfer et al., 2011; Schumann et al., 2011). In our case, it is likely that the effective
separation of SO2 from the ash particle plume transported to
our measurement was the main cause of the lack of additional
sub-micron particles.
Mass concentration of major inorganic ions in the four
super-micron size fractions measured using the MOUDI impactor during the late part of the episode are shown in Fig. 6.
The total concentration of inorganic ions in the sample was
about 9 µg m−3 . Sodium and chloride indicative of sea-salt
particles explained most of this mass, and the next abundant
ion was nitrate (about 1.5 µg m−3 ) followed by ammonium
(1.0 µg m−3 ) and non-sea-salt sulfate (0.6 µg m−3 ). The estimated sea-salt mass concentrations exceeded the average
sea-salt mass concentration at the Helsinki area during the
spring/summer period by more than a factor 20 (see Kerminen et al., 2000). The probable reason for such a high
amount of sea salt in our sample was the dominance of marine air masses combined with relatively strong wind speeds.
Altogether, the analyzed ions explained less than 20% of the
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V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
2011/05/24 12:00 UTC
80°N
70°N
60°N
60°N
50°N
50°N
20°W
10°W
0°
10°E
20°E
30°E
2011/05/25 12:00 UTC
80°N
20°W
70°N
60°N
60°N
50°N
50°N
1
10°W
0°
10°E
20°E
2
5
10
20
30°E
50
10°W
0°
10°E
20°E
30°E
2011/05/26 00:00 UTC
80°N
70°N
20°W
2011/05/25 00:00 UTC
80°N
70°N
12233
20°W
10°W
0°
10°E
20°E
30°E
100 200 500 1000 5000
Fig. 4. The SILAM dispersion model results for the column-integrated volcanic ash concentration (mg m−2 ) during 24–25 May 2011.
1.0
-3
dM/dlogDa (µg m )
40
30
Sodium
Chloride
Nitrate
Ammonium
Sulfate
Potassium
0.8
-3
Before episode
Volcanic eruption
episode
After episode
Concentration (µg m )
50
0.6
0.4
0.2
20
0.0
MOUDI 6 (1-1.8 µm)
10
0
0.1
1
10
100
MOUDI 7 (1.8-3.2 µm)
MOUDI 8 (3.2-5.6 µm)
MOUDI 9 (5.6-10 µm)
Fig. 6. Concentrations of sodium, chloride, nitrate, ammonium,
sulfate and potassium in the four size ranges obtained by the IC
measurements from the MOUDI sample that was collected between
15:15 and 17:00 during the episode on 25 May.
Da (µm)
Fig. 5. APS-derived mass size distributions of super-micron particles before the volcanic episode (03:00–06:00 local summer time
on 25 May), during the episode (06:30–17:30), and after it (18:00–
21:00). A density of 2.6 g cm−3 has been assumed in converting the
APS data.
super-micron particulate matter. Our earlier measurements
suggest that besides sea salt, the super-micron size range is
likely to include road dust, other mineral particles, and primary biogenic particles in the Helsinki area (e.g. Pakkanen
et al., 2001; Viidanoja et al., 2002). As will be shown in
www.atmos-chem-phys.net/11/12227/2011/
Sect. 3.3, the great majority of super-micron particulate matter during the episode on 25 May was volcanic ash rather than
other primary particles more typical to our measurement site.
Additional information on the origin and atmospheric processing of super-micron particles can be obtained by looking at the relative abundances of the measured inorganic ions
in different size fractions. We calculated that about half
of the chloride originally present in sea-salt particles had
been replaced with other acidic substances, mostly nitrate
and sulfate, before entering our measurement site. Excluding the largest size range, the extent of chloride depletion
Atmos. Chem. Phys., 11, 12227–12239, 2011
12234
V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
-3
Concentration (µg m )
20
15
Organics
Sulphate
Nitrate
Chloride
Ammonium
10
5
1.0
Contribution
0.8
0.6
0.4
0.2
0.0
22/5/11
23/5/11
24/5/11
25/5/11
26/5/11
27/5/11
28/5/11
Fig. 7. Mass concentrations (top panel) and relative abundances
(bottom panel) of particulate organic matter, sulfate, nitrate, ammonium and sodium measured by the ACSM in the PM1 aerosol
particles between 22 and 28 May 2011.
decreased with increasing particle size similar to what has
been observed in a large number of field experiments around
the world (see, e.g. Kerminen et al., 1998, and references
therein). Consistent with the surface reaction of gaseous nitric acid with sea-salt or other super-micron particles, the
mass fraction of nitrate of the all measured inorganic ions
decreased strongly with increasing particle size, being equal
to 26, 20, 9 and 2 % in the four size ranges. Non-sea-salt
sulfate was relatively evenly distributed between particles of
different size. The size range 5.6–10 µm contained very little
chloride but quite large mass fractions of ammonium, potassium and non-sea-salt sulfate. This feature suggests that particles other than sea salt had been present, and that these
particles had interacted with sea-salt during the atmospheric
transportation.
Next, let us have a look at the chemistry of sub-micron
particles. The top panel of Fig. 7 shows the concentrations of
compounds measured by the ACSM between 22 and 28 May
2011. We may see that nitrate and particulate organic matter (POM) concentrations in PM1 were lower during the volcanic episode than during most other times of the considered
time period. Contrary to this, sulfate concentrations were at
their highest during the episode. The enrichment of sulfate
in PM1 during the episode is evident when looking at the relative abundance of the compounds measured by the ACSM
(bottom panel of Fig. 7).
The annual-average concentrations of sulfate and POM
(assumed to be 1.6 times the organic carbon concentration) in
PM1 have been observed to be about 1.5 and 2.4 µg m−3 , respectively, at our measurement site (Saarikoski et al., 2008).
Based on this and the findings above, it might be tempting to
Atmos. Chem. Phys., 11, 12227–12239, 2011
conclude that a big fraction of the sulfate in PM1 and nonsea-salt sulfate in super-micron particles was of volcanic origin during the 25 May episode. Before drawing this conclusion, however, we need to keep in mind that also sources
other than volcanoes are able to result in high sulfate-toPOM concentration ratios. Timonen et al. (2008) found that
sulfate-to-POM concentration ratios comparable to that during the volcanic episode on 25 May are frequently observed
in air masses coming from continental Europe or Russia to
our measurement site. In air masses originating from marine
areas, such as the air measured practically the whole time period between 22 and 28 May, sulfate concentrations are typically lower than POM concentrations (Timonen et al., 2008).
Similar to the PM1 mass concentrations, the scattering and
absorption coefficients of sub-micron particles did not show
any notable increase during the episode (data not shown).
The removal of the PM1 inlet from the nephelometer for
three hours during the episode resulted in the increase of the
aerosol scattering coefficients by factors of 3–4, presumably
due to high concentrations of super-micron particles at that
time. Even after removing the inlet, the aerosol scattering
coefficients were lower than during the periods of the highest PM1 concentrations encountered between 22 and 28 May.
Measured visibility anti-correlated with the aerosol scattering coefficient and showed a minor decline during the volcanic episode. We may conclude that a volcanic air pollution episode of the magnitude observed here causes an optical signal that may be difficult to be distinguished from the
“normal” variability in aerosol optical properties caused by
changes in concentrations other pollutants present in continental boundary layers.
3.3
Individual particle analysis
Both the physical and chemical aerosol properties discussed
in the previous section are fully consistent with the entrance
of volcanic material at our measurement site on 25 May.
However, based solely on the measurements considered so
far, we cannot entirely exclude the possibility that this
episode was mainly or partly caused by a suitable combination of some other particle types, such as biological particles, soil dust or road dust mixed with anthropogenic pollution. By relying on individual particle analysis, we will next
show that a very big fraction of super-micron particles observed during the event were indeed volcanic ash, and that
some volcanic material was also present in the sub-micron
size range.
A total of 193 particles collected on TEM grids during the
episode were exposed to the TEM/EDX individual particle
analysis. In addition to the elemental spectra, we recorded
the size, morphological information and susceptibility to
damage by an electron beam for all these particles. The particles were classified into different particle clusters (= groups)
based on their major elements (Na, Mg, Al, Si, P, S, Cl, K,
Ca, Ti and Fe) using hierarchical cluster analysis (Ward’s
www.atmos-chem-phys.net/11/12227/2011/
V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
12235
more, very minor amounts of K, P, Cl and S were observed
in some particles. We compared the elemental ratios of group
80
1 with the corresponding values reported for Grimsvötn vol70
canic ash fallout collected from Iceland during the eruption
60
on 22 May 2011 (Oskarsson and Sverrisdóttir, 2011). These
50
two sets of the elemental ratios were very close to each other,
40
which confirm that particles in group 1 represent “pure” vol30
canic ash from the Grimsvötn eruption in the sense that the
20
ash particles had not been interacted with other type of particles before entering our measurement site. The elemen10
tal composition of ash particles was very homogenous, as
0
0.2-0.5 µm 0.5-1.0 µm 1.0-1.8 µm 1.8-11 µm
typical for volcanic ash from Grimsvötn tholeiite-basalt rock
n=26
n=38
n=46
n=59
(Oskarsson and Sverrisdóttir, 2011).
Figure 8. Relative abundances (%) of 14 particle groups in the four size fractions
Fig. 8. by
Relative
abundances
(%)particle
of 14analysis
particle
groups
in thesample
four that was In general, the 14 particle groups chosen in this paper repobtained
the TEM/EDX
individual
from
the Battelle
collected
betweenobtained
11:37 andby
15.05
summer time
during the
episodeanalyon 25 May. resent particles ranging from “pure” ash (group 1) to ash
size fractions
thelocal
TEM/EDX
individual
particle
mixed with sea salt (groups 2-5), sea salt particles with varisis from the Battelle sample that was collected between 11:37 and
15:05 local summer time during the episode on 25 May.
able degrees of Cl substitution by sulphate or nitrate due
to interaction with trace gases (groups 6–8), particles containing mostly secondary material, such as ammonium, sulO
Si
phate, nitrate and organic compounds (groups 9-12), and
combustion-derived primary particles, such ash soot and Ferich particles with secondary material (groups 13–14). Examples of different particle types are illustrated in Fig. 10.
Volcanic ash particles were totally beam-resistant and their
shapes were very variable from angular oval particles to anNa Al
gular flakes or rods, containing sometimes very sharp edges.
Cu
The submicron secondary particles were easily damaged unCa
Fe
C O Mg
der intentional, strong electron beam, as typical for these parTi
S
K
Ca
ticle types (Niemi et al., 2006).
Figure 8 summarizes the relative abundances of the different particles types in the size ranges 0.2–0.5 µm, 0.5–1.0 µm,
X-ray energy (keV)
1.0–1.8 µm and 1.8–11 µm. We may see that the largest size
range contained almost exclusively “pure” volcanic ash partiFig. 9. An EDX spectrum of a typical particle from the group 1,
Figure 9. An EDX spectrum of a typical particle fromcles
the(∼20%)
group 1,
pure volcanic
ash particles (∼30 %),
andi.e.
relatively
fresh sea-salt
i.e. pure volcanic ash particle. C and Cu peaks originate from TEM
particle.
C
and
Cu
peaks
originate
from
TEM
grid
substrate.
or mixtures of these two particle types (∼50 %). The size
grid substrate.
range of 1.0–1.8 µm resembled the largest size range, but
with the difference that there were fewer ash particles and
method, Squared Euclidean distances; IBM SPSS Statistics
almost no particles that could be termed as “pure” ash. The
19 program). We tested for a different number of clusters
size range of 0.5–1.0 µm was dominated by reacted sea-salt
and ended up to 13 groups. Carbon results were not used
particles followed by ash particles. In this size range, also
in cluster analysis due to its presence in the carbon-Formvar
small numbers of primary combustion and secondary particoating of TEM-grids. However, C-rich soot particles were
cles appeared. The smallest size range was dominated by
indentified before cluster analysis based on their morphology
secondary particles and primary combustion particles, and it
and high C intensity compared with X-ray spectra analyzed
had a small fraction of reacted sea salt particles. After more
from blank carbon-Formvar coating. Thus, the soot partiaccurate inspection of the TEM grids and subsequent seleccles formed the 14th particle group. Other C-rich primary
tive analysis, we were able to found ash particles also from
particles, such as tar balls from biomass burning or various
this smallest size range. Their abundance in relation to other
biological particles (Niemi et al., 2006), were not observed
particles types was, however, almost negligible. Figure 11
in the 0.2–11 µm size range.
demonstrates the smallest volcanic ash particle from the 0.2–
The 14 particle groups and their major elements (exclud0.5 µm sample and the largest ash particle from the >11 µm
ing O, N, C and H) in the four size fractions are shown in
sample that contained only two volcanic ash particles and
Fig. 8. Particles in group 1 were mainly in the 1.8–11 µm
one primary biological particle.
size fraction and contained abundant Si (mean 42 % of XThe presence of substantial amounts of mixed ash-sea salt
ray counts), Fe (14 %), Al (13 %), Ca (12 %) and Mg (7 %)
particles indicates that the volcanic ash had entrained into
as well as some Na (4 %) and Ti (3 %) (Fig. 9). Furtherthe boundary layer air well before entering our measurement
100
Relative abundance (%)
90
1. Si-Fe-Al-Ca-Mg-Na
2. Si-Fe-Na-Al-Cl-Ca-Mg
3. Si-Cl-Na-Fe-Al-Ca-Mg
4. Cl-Na-Si-Al-Fe-Ca-Mg
5. Na-Cl-Si-Fe-Mg-S-Ca
6. Cl-Na-S-Mg
7. Na-S-Mg-Cl-Ca
8. Na-Mg-S
9. S-Si-Ca-Na-K-Al-Fe
10. S-Na-Mg
11. S
12. K-S
13. Fe-S
14. C-soot
www.atmos-chem-phys.net/11/12227/2011/
Atmos. Chem. Phys., 11, 12227–12239, 2011
12236
V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
PM 1.8−11 μm: Pure volcanic ash (1), mixed volcanic ash
and sea salt (2−4), sea salt (6).
3
6
6
1
200 nm
4
6
10 µm
Fig.
11. TEM
volcanicof
ashvolcanic
mixed withash
secondary
Figure
11.images
TEMofimages
mixedmatewith seconda
rial
in
the
0.2–0.5
µm
sample
(left
image)
and
volcanic
ash
in
the sample (r
sample (left image) and volcanic ash in the >11 µm
>11 µm sample (right image) collected between 11:37 and 15:05
between
and 15:05
local
summer
local
summer11:37
time during
the episode
on 25
May. time during the episo
2
5 μm
PM 1.0−1.8 μm: Mixed volcanic ash and sea salt (2−3),
sea salt (6).
2
6
3
6
2 μm
PM 0.5−1.0 μm: Sea salt with high level of Cl depletion
(7−8).
8
8
7
8
7
1 μm
PM 0.2−0.5 μm: Secondary particles (10, 11), Fe-rich with
secondary material (13), soot particle (14).
10
11
site. The entrainment could not be due to gravitational settling alone, since settling velocities of sub-micron ash particles are expected to be very low. One could note that submicron ash particles in surface air were also detected in association with the Eyjafjallajökull eruption in April, 2010 (Lettino et al., 2011).
Our individual particle analysis unambiguously demonstrates that volcanic ash particles were present at sizes from
less than 0.5 µm up to sizes >10 µm at our measurement site
during the May 25 episode. Together with sea-salt, ash appeared to account for the great majority of super-micron particles in terms of particle number. Estimating the exact contribution of ash to super-micron particulate mass is complicated due to their variable sizes and partial mixing with sea
salt and other particle. Based on their big share in the collected sample, however, it is very likely that volcanic was the
main component of super-micron particle mass during the
episode, enhancing PM10 mass concentrations up to several
tens of µg m−3 . Such an enhancement is comparable to observed maximum increases of PM10 mass concentrations in
France and Germany after the Eyjaflallajökull eruption (Colette et al., 2011; Schäfer et al., 2011).
4
Conclusions
11
The investigation made in this paper has demonstrated the
power of combining surface aerosol measurements, dispersion model simulations and satellite measurements in detecting and analyzing surface air pollution episodes caused
14
by long-range transported material from volcanic eruptions.
11
None of these three approaches alone would be sufficient for
13
μm
this purpose. Satellite measurements, while easily detecting
1
a major volcanic plume in cloud-free air, become less accurate in following weak and diluting plumes further away from
Fig. 10. TEM images of typical particle types in four size fracvolcanoes. Furthermore, most satellites are not yet able to
tions from the Battelle sample that was collected between 11:37
provide detailed information about the size and vertical locaandFigure
15:05 local
time during
episode
on 25 May.
The
10. summer
TEM images
of the
typical
particle
types
in four
size
fractionsparticles.
from the
Battellemodel calculations are
tion
of volcanic
Dispersion
numbering
particle
is the same
as in Fig.
8. and 15:05 local summer time during the episode
sample of
that
wastypes
collected
between
11:37
extremely useful in following the atmospheric transportation
on 25 May. The numbering of particle types is the same as in Fig 8.
Atmos. Chem. Phys., 11, 12227–12239, 2011
www.atmos-chem-phys.net/11/12227/2011/
V.-M. Kerminen et al.: Characterization of a volcanic ash episode in southern Finland
of volcanic material, but have challenges in predicting the entrainment of this material into the boundary layer air. Routine air pollution monitoring reveals the presence of significant amounts of volcanic material in the surface air but,
without complementary and very detailed aerosol measurements, cannot unambiguously tell us whether elevated pollutant concentrations are due to volcanic emissions or some
other sources.
Similar to the detection of volcanic material in the surface air, forecasting of air pollution episodes caused by volcanic eruptions requires the complementary use of remote
sensing, dispersion model simulations, and surface air monitoring. The first two of these methods are needed to identify
and foresee the situations in which volcanic material might
have transported over different locations. By combining this
information with data from a sufficiently large and near realtime air pollution network, people susceptible to air pollution
will get some time to prepare themselves for the arrival of
volcanic pollution.
Acknowledgements. This study has been funded by the Academy of
Finland (projects IS4FIRES and ASTREX), by the CLEEN/MMEA
program and the KASTU-2 project (decision number 40396/10)
of the Finnish Funding Agency for Technology and Innovation
(TEKES), and by EU FP7 MACC projects. We would also like to
thank Electron Microscopy Unit of the Institute of Biotechnology,
University of Helsinki, for providing laboratory facilities. The authors gratefully acknowledge the NOAA Air Resources Laboratory
(ARL) for the provision of the HYSPLIT transport and dispersion
model and/or READY website (http://www.arl.noaa.gov/ready.php)
used in this publication.
Edited by: M. C. Facchini
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